This study is to define ethnic differences in post-synaptic mechanisms of vascular response to adrenergic agonists, and in particular to localize the sites previously identified attentuation of vasodilation to isoproterenol in African Americans. Also, th e effects of ethnicity on central and peripheral mechanisms of the adrenergic control and to define the sites of such differences and their regulation will be studied. This study has been expanded to include the effects of estrogen on vascular reactivity.